All cellular events occur with the expenditure of energy. The source of this energy is the foodstuffs ingested by the organism. In most organisms, the stored chemical energy is converted to work and other chemical compounds. In bioluminescent organisms, some of the chemical energy is emitted as light. The general pathway leading to light appears to be the reaction between a reduced compound, (a) "luciferin") and usually molecular oxygen in the presence of an enzyme (a "luciferase"). The chemical identity of the luciferin varies from species to species and the various luciferases are distinctly different. The details of the chemistry of bioluminescence from a group of bacteria has recently been sufficiently well established to examine the changes in thermodynamic parameters accompanying photon emission. By appropriate calorimetric methods, the enthalpy of bacterial bioluminescence is being studied both in vitro and in vivo. Model bioluminescent reactions are also being studied. The quaternary structure of enzymes plays a key role in regulation. The quaternary structure can be described by a reversible equilibrium reaction and thermodynamic parameters ascribed to it. Bacterial luciferase is composed of two subunits. Work is currently in progress to study to reversible association and determine the magnitude of the association constant by ultracentrifugal methods.